Device and process for controlling the pressure and delivery of a fuel feed to a servovalve unit

ABSTRACT

A device for controlling fuel pressure and delivery as necessary for the operation of the servovalves of an aircraft engine is disposed downstream of a fuel injector feed unit of the engine. The device comprises a chamber maintained at the servovalve feed pressure by the movement of a closure member to vary the opening of a port of the chamber. The closure member moves under the action of opposing forces comprising inter alia a force in the opening direction proportional to the servovalve feed pressure. An equilibrium position of the closure member is a function of pressure resulting from the engine speed. An injector delivery control loop is therefore isolated from a servovalve pressure and delivery control loop.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates generally to hydromechanical devices formetering the delivery and pressure of a fuel feed to a hydraulicservovalve unit which is driven by the fuel. The invention also relatesto a fuel supply circuit comprising said device and to a process forcontrolling the pressure and delivery of the fuel to a servovalve unit.

[0003] 2. Technological Background

[0004] In an aircraft engine the fuel is supplied to fuel injectors atwhich the fuel is burned to provide the propulsive power and the powerfor feeding engine auxiliaries. The fuel is also used as a coolingliquid in heat exchangers, and also to operate a plurality of hydraulicservovalves. For example, nozzles and other variable-geometry elementsare operated by hydraulic rams which are driven by a supply of the fuelunder pressure.

[0005] The servovalves are usually supplied from a central servovalvefeed unit whose pressure must remain constant and whose delivery must becapable of being varied in accordance with servovalve requirements.

[0006] The supply circuit for the injectors, exchangers and centralservovalve feed unit comprises a low pressure pump which raises fuelcoming from the aircraft tanks at an initial pressure Pca to a pressurePb, a high pressure pump which further raises the fuel to a pressurehigher than that required for the injectors and the servovalve feedunit.

[0007] Unused fuel is recycled. The proportion of recycled fuel relativeto consumed fuel must not be excessive-since the fuel is heated by itspassage through the high pressure pump and is therefore less suitable asa cooling liquid. The high pressure pump may be a pump whose delivery isdetermined solely by its speed of rotation. This speed is in turn afunction of engine speed. The pump may also be a pump whose delivery isa function not only of its speed but also of another parameter which canbe so controlled that a pump running at a given speed can providedifferent delivery rates.

[0008] As explained in U.S. Pat. No. 5,715,674 to Reuter, variabledelivery pumps have the advantage over fixed pumps of being able todeliver a fuel flow rate theoretically adapted to the instantaneousdelivery required for engine operation.

[0009] The term “theoretically” is used because, as explained in thesaid Reuter patent, the pump response time is not negligible. Moreparticularly, the servovalves introduce erratic delivery variationsrequiring a control bandwidth at frequencies incompatible with pumpcontrol capacities whether the pump is of the variable geometry kind orof the fixed delivery kind. Excessive response times may lead todecrease in deliveries to the engine fuel injectors and/or malfunctionof the servomechanisms operated by the pressurised fuel.

[0010] To overcome this problem the Reuter patent provides a pumpcontrol valve having an outlet connected to an element for controllingthe delivery of the pump. The operation of the valve is not explainedand, in particular, it is not clear how the pump can provide a bettertransient response time as stated in the last two lines of column 6 andthe first line of column 7 of Reuter. It is clear, however, that thevalve is an essential element of a control loop aimed at reducing asmuch as possible the delivery of the pump. This delivery istheoretically maintained at a level slightly above the level necessaryto cover the delivery demanded by the engine fuel injectors and thedelivery demanded at any instant by the servovalves.

[0011] Although such a device is an improvement over the prior art itdoes not provide a response time short enough to meet the requirementsof the servovalves controlling various elements of the engine.

SUMMARY OF THE INVENTION

[0012] The inventors have realised that the response time of variabledelivery pumps is short enough for a control loop comprising such a pumpto control the fuel delivery to the engine fuel injectors without majorproblems. The latter delivery varies in accordance with known laws independence upon engine speed and other parameters such as temperatureand the external pressure, which it is possible to monitor continuouslyso that they are known at any instant. Since the engine is a highinertia device its speed of rotation varies slowly. Similarconsiderations apply to the other parameters affecting the deliveryconsumed by the injectors.

[0013] The invention is therefore based on isolating the injectordelivery control loop from the servovalve feed control loop.

[0014] To this end, the delivery required from the high pressure pump atany instant needs to be the sum of the delivery required by the enginefuel injectors at that instant plus the maximum delivery which may berequired by the servovalves.

[0015] The maximum servovalve delivery is the delivery which would benecessary if all the servovalves were simultaneously each to demand themaximum delivery consumable by that servovalve. The sum of the maximumdeliveries to all the servovalves for a given engine speed is thereforea known constant, so that the delivery variations of the pump are solelythe variations of the fuel injector consumption. Delivery to the fuelinjectors is therefore readily controllable.

[0016] When the required portion of the pump delivery has been deliveredto the fuel injectors, the remainder of the delivery is sufficient tomeet all servovalve requirements in all circumstances.

[0017] The invention therefore provides a pressure and delivery controlloop for fuel directed to a servovalve feed unit downstream of a fuelinjector feed unit, comprising a fuel intake at a first pressure and ata constant controlled delivery and a control device for keeping thepressure of the feed unit constant whatever the consumption of said unitand the variations of the various fuel pressures in dependence upon theengine speed and upon the other parameters affecting the pressures.

[0018] In an embodiment of the invention the pressure control devicecomprises a chamber for providing this constant pressure. The chambercommunicates with the feed unit and, by way of a variable cross-sectionoutlet port, with a portion of the fuel circuit between the highpressure pump and the low pressure pump.

[0019] The cross-section of the outlet port is varied in dependence uponthe position of a first shutter which moves around a variableequilibrium position. This position is a function of the pressuredifference between a first pressure downstream of the low pressure pumpand upstream of the high pressure pump and a second pressure upstream ofthe low pressure pump. The movements of the first shutter around thevariable equilibrium position are a function of the fuel consumed by theservovalve feed unit.

[0020] The variation of the port cross-section around the variableequilibrium position is effected, as explained above, by means of afirst shutter movable in response to two opposing forces. One of theseforces is provided by the combined action of the second pressure and ofresilient means, such as a spring, and the other force is provided bythe action of the first pressure on the shutter.

[0021] Variation of the port cross-section around the equilibriumposition in dependence upon feed unit consumption is effected by themovement of a second movable shutter which is movable in response to twoopposing forces. One of these forces is provided by the pressure forcesexerted by the fuel combined with resilient forces created, for example,by a spring. The other force is created by fuel pressure forces. One ofthese other forces is created by fuel at the pressure of the servovalvefeed. Consequently, a decrease in the latter pressure reduces the latterforce and therefore leads to a closing movement of the second shutter.

[0022] For the sake of simplicity and improved reliability, the twoshutters of the chamber are merged to form a single element formed bypart of a movable spool which variably masks the opening of the outletport. The spool has two ends and an intermediate part forming a piston.

[0023] The spool piston divides a compartment into two half-chambers,namely a first half-chamber and a second half-chamber, the latter beingat the controlled servovalve feed pressure. The first half-chambercommunicates with the upstream side of a diaphragm, and the secondhalf-chamber communicates with the downstream side of the diaphragm. Theupstream side of the diaphragm receives fuel originating from the highpressure pump, preferably through a filter. A spring applies a pressureto the spool in a direction towards the second half-chamber.

[0024] The first end of the spool is subjected to the pressure Pca andits opposite end is subjected to the pressure Pb.

[0025] The forces acting on the spool and tending to reduce the openingof the outlet port are:

[0026] the force exerted by the pressure Pca acting on the first end ofthe spool;

[0027] the force of the spring; and

[0028] the force exerted by the pressure upstream of the diaphragm andacting on the spool piston.

[0029] The opposing forces acting on the spool and tending to increasethe opening of the outlet port are:

[0030] the force exerted by the pressure Pb acting on the second end ofthe spool; and

[0031] the force exerted by the pressure downstream of the diaphragm andacting on the spool piston.

[0032] The parameters on which a direct action is possible by design andwhich must be considered in order to obtain a constant controlledpressure in dependence upon servovalve delivery are:

[0033] the cross-sectional areas of the first and second ends of thespool;

[0034] the diaphragm cross-section;

[0035] the elastic coefficient of the spring; and

[0036] the area of the spool piston.

[0037] In summary, according to the invention, in a fuel supply circuitof an aircraft engine including fuel injectors and a plurality ofservovalves, a low pressure pump for raising the pressure of fuel from atank from a pressure Pca to a pressure Pb, and a high pressure pump andfilter for further raising the pressure of the fuel to a pressure Psf atthe outlet of said filter, there is provided a device for controllingthe pressure and delivery of fuel to said plurality of servovalvescomprising:

[0038] a compartment divided in a sealed manner to define a first endchamber, an intermediate chamber, and a second end chamber;

[0039] a spool movable in said compartment and having first and secondend portions and an intermediate portion;

[0040] said first end portion being engaged in a sealed manner in saidfirst end chamber;

[0041] said intermediate portion including a piston dividing saidintermediate chamber in a fluid tight manner into first and secondhalf-chambers;

[0042] means for communicating said first end chamber with fuel at thepressure Pca;

[0043] means for communicating said second end chamber with fuel at thepressure Pb;

[0044] a first outlet port in said intermediate chamber in communicationwith fuel at said pressure Pb;

[0045] resilient means disposed in said first half-chamber and acting tourge said spool in the direction of said second end chamber;

[0046] diaphragm means having an inlet and an outlet;

[0047] said inlet being connected to receive fuel at the pressure Psfand also being connected to said first half-chamber;

[0048] said outlet being connected to a fuel feed line to saidservovalves and also being connected to said second half-chamber;

[0049] said spool being movable in said compartment in response toforces exerted on said spool by said resilient means and the fuelpressures applied to said first and second end chambers and to saidfirst and second half-chambers to cause said piston to vary the openingof said first outlet port.

[0050] Preferably, the cross-sectional areas of the first and secondends of the spool are identical. Consequently, the force tending toincrease the outlet port opening is proportional to the pressuredifference Pb-Pca. This pressure difference is in turn proportional tothe square of the engine speed. The delivery through the diaphragm isproportional to the pressure upstream thereof.

[0051] Consequently, the delivery passing through the control device andconsisting of the sum of the delivery to the servovalves and the returndelivery from the exit of the variable outlet port of the secondhalf-chamber is proportional to engine speed, at least within a certainrange. This characteristic is well adapted to the delivery necessary forthe servovalves, whose requirements increase with engine speed.

[0052] The invention also provides a process for controlling thepressure and delivery of fuel to said plurality of servovalves of anaircraft engine, comprising the steps of:

[0053] controlling the high pressure pump delivery so that, whatever theengine speed, the delivery from said high pressure pump corresponds tothe sum of the delivery required by said fuel injectors at said enginespeed and the prevailing flight conditions and the delivery which wouldbe required by said plurality of servovalves if all of said servovalvesoperate simultaneously using the maximum delivery usable at said enginespeed;

[0054] placing a servovalve feed unit downstream of a fuel injector feedunit and in communication with a compartment having an opening which isvariable in dependence upon the position of a movable closure member;and,

[0055] causing said closure member to move to vary said opening inresponse to opposing first and second groups of forces, said first groupof forces acting in the closing direction and comprising a resilientforce, a force proportional to the pressure Pca, and a forceproportional to a first pressure, and said second group of forces actingin the opening direction and comprising a force proportional to thepressure Pb, and a force proportional to a second pressure less than thefirst pressure and at a substantially constant difference therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

[0056]FIG. 1 is a schematic diagram of an aircraft engine fuel supplycircuit incorporating an embodiment of a servovalve delivery controldevice in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0057] The fuel supply circuit of FIG. 1 is shown in very simplifiedform, only the connections of the servovalve control device being shown,elements such as filters, heat exchangers and bypasses which areconventionally used in aircraft fuel supply circuits being omitted.

[0058] The fuel arrives from an aircraft fuel tank via a line 1 at apressure Pca. Its pressure is raised by a low pressure feed pump 2 whichdelivers fuel at a pressure Pb. The fuel leaving the pump 2, togetherwith fuel from a return line which will be referred to hereinafter, aredelivered via a line 21 to a high pressure pump 3 whose output isadjustable according to the drive speed.

[0059] In accordance with the invention, the controlled delivery fromthe pump 3 is equal to the sum of the delivery needed to supply theengine fuel injectors and the maximum possible delivery which may beconsumed by the hydromechanical devices which use the pressurized fuelas the hydraulic driving fluid. Consequently, the delivery variations tobe accommodated by the pump 3 are merely the variations in the deliveryto the engine fuel injectors. These variations are slow enough to becontrolled without difficulty. More particularly, they are compatiblewith the pump response time.

[0060] The fuel from the pump 3 passes through a line 22 to aself-cleaning filter 4 which, in known manner, has an outlet 5 forunfiltered fuel and an outlet 6 for filtered fuel.

[0061] The unfiltered fuel is delivered to a unit 9 for metering fuel tothe engine fuel injectors. The filtered fuel from the outlet 6 isdirected through a heater 7 before being supplied for use by servovalvesoperating mechanisms of the engine.

[0062] In accordance with the invention a device 10 is provided tomaintain a constant servovalve feed pressure whatever delivery is takenby the servovalves which drive the engine mechanisms.

[0063] The device 10 therefore receives, via line 8, a fuel deliverywhich is substantially equal to the maximum possible delivery requiredif all of the servovalves are to be operated simultaneously. Theproportion of the fuel actually required by the servovalves is deliveredby the delivery control device 10 through a line 23 to the servovalves,and the unused proportion is returned via a line 24 to the line 21feeding the high pressure pump 3 as previously stated.

[0064] The servo delivery control device basically comprises acompartment 11 divided into three chambers which are sealed from oneanother, namely a top chamber 12, an intermediate chamber 13, and abottom chamber 14. The words “top” and “bottom” are used merely todifferentiate the chambers from one another. In the actual equipment thechambers can have positions relative to one another which are differentas regards their respective levels. If preferred the top and bottomchambers may be referred to as first and second chambers respectively.

[0065] In this embodiment a spool 30 is slidably mounted in thecompartment 11, the spool 30 comprising a rod 31 having a shoulder 33forming a piston in the intermediate chamber 13. The top end 32 of therod extends sealingly into the top chamber 12, and the bottom end 34 ofthe rod extends sealingly into the bottom chamber 14. The intermediatechamber 13 is divided by the piston 33 into two half-chambers, namely anupper intermediate half-chamber 15 and a lower intermediate half-chamber16.

[0066] The filtered fuel delivered to the device 10 via line 8 has apressure Psf and is supplied to the upper half-chamber 15 and to theinlet of a diaphragm device 17. The outlet of the diaphragm device 17communicates with the servovalve feed line 23 and with the lowerhalf-chamber 16 though a port 18 thereof.

[0067] Fuel not used by the servovalves exits the lower half-chamber 16through a variable cross-section outlet port 19 for recycling via lines24 and 21. The opening of the outlet port 19 is controlled by movementof the piston 33 within the intermediate chamber 13.

[0068] A spring 35 is disposed in the upper half-chamber 15 and bears onthe piston 33 to apply thereto a force tending to move it towards thelower half-chamber.

[0069] The position adopted by the piston 33 in the intermediate chamber13 is a function of the force exerted by the spring 35, the pressure Pcaof the fuel acting on the top end 32 of the spool 30, and the pressurePb of the fuel acting on the bottom end 34 of the spool 30. Also, thepressure difference Pb-Pca is proportional to the square of the numberof engine revolutions since the pumps 2 and 3 rotate at speedsproportional to the number of engine revolutions per minute.

[0070] Also, the diaphragm 17 maintains a constant pressure differencebetween its inlet and outlet sides, and the position of the spool 30 isalso controlled by this pressure difference because the pressure Psfacts on the top surface of the piston 33 and the pressure at the outletof the diaphragm 17 acts on the bottom surface of the piston 33.

[0071] In the equilibrium state the pressure in the bottom chamber isequal to the pressure necessary to feed the servovalves—i.e., the fueldelivery arriving through the diaphragm 17 and the fuel delivery Qcescaping through the port 19 of the intermediate chamber 13 are such asto produce the latter pressure.

[0072] Let us now assume that, with all other factors remaining thesame, in particular the engine speed and therefore the pressures Pb andPsf, a servovalve calls for fuel. This will cause the pressure in thelower half-chamber 16 to drop, and the pressure Psf in the upperhalf-chamber 15 and the spring 35 will urge the piston 33 downwards withthe result that the area of the port opening 19, and therefore the flowtherethrough, is reduced until a new equilibrium state is reached. Inthe case of a pressure increase the converse phenomenon (upward movementof the piston) leads to an increase of the delivery through the port 19.

[0073] Clearly, therefore, from a variable equilibrium positiondependent upon the pressures supplied by the pumps, the spool will moveto keep the pressure available for the servovalves constant whatever theservovalve demand may be.

[0074] Preferably, safety elements are provided to ensure that possibledefects do not lead to impaired operation. For example, an overpressurerelief valve 20 is placed in parallel with the diaphragm 17 and willopen in the event of the diaphragm 17 becoming clogged or icing up, thuslimiting the pressure drop across the diaphragm.

[0075] A mechanical stop 26 limits the upwards travel of the spool30—i.e. movement in a direction which increases the opening of theadjustable port 19. The stop 26 will limit the permissible returndelivery at the exit of the bottom half-chamber in the event of thepressure difference Psf-Pb dropping below a minimum value. The stop 26will therefore guarantee a delivery margin to the engine fuel injectorsbetween speeds ranging from idling to full throttle operation, but couldnot guarantee this margin for in-flight re-ignition.

[0076] Preferably, the stop 26 is in the form of a screw introduced intoa tapped aperture parallel to the direction of spool movement. Theposition of the stop 26 can then be adjusted.

[0077] In an embodiment which also provides improved control of pumpbearing cooling, the bottom half-chamber has a second outlet port 29 forpartially supplying the cooling exchanger of the pump bearings. The port29 is so positioned as to be closed i.e., completely masked by thepiston 33—at low engine speeds—i.e., the port 29 is completed masked bythe piston 33 when the opening of the port 19 is reduced.

[0078] When engine speed increases, the pressure Pb increases and theequilibrium position of the piston 33 rises, thus releasing a deliveryof fuel for pump bearing cooling. Suppressing the cooling delivery atlow engine speeds leads to readier optimisation of pump bearing coolingat high speeds.

We claim:
 1. In a fuel supply circuit of an aircraft engine includingfuel injectors and a plurality of servovalves, a low pressure pump forraising the pressure of fuel from a tank from a pressure Pca to apressure Pb, and a high pressure pump and filter for further raising thepressure of the fuel to a pressure Psf at the outlet of said filter, adevice for controlling the pressure and delivery of fuel to saidplurality of servovalves comprising: a compartment divided in a sealedmanner to define a first end chamber, an intermediate chamber, and asecond end chamber; a spool movable in said compartment and having firstand second end portions and an intermediate portion; said first endportion being engaged in a sealed manner in said first end chamber; saidintermediate portion including a piston dividing said intermediatechamber in a fluid tight manner into first and second half-chambers;means for communicating said first end chamber with fuel at the pressurePca; means for communicating said second end chamber with fuel at thepressure Pb; a first outlet port in said intermediate chamber incommunication with fuel at said pressure Pb; resilient means disposed insaid first half-chamber and acting to urge said spool in the directionof said second end chamber; diaphragm means having an inlet and anoutlet; said inlet being connected to receive fuel at the pressure Psfand also being connected to said first half-chamber; said outlet beingconnected to a fuel feed line to said servovalves and also beingconnected to said second half-chamber; said spool being movable in saidcompartment in response to forces exerted on said spool by saidresilient means and the fuel pressures applied to said first and secondend chambers and to said first and second half-chambers to cause saidpiston to vary the opening of said first outlet port.
 2. A deviceaccording to claim 1, further comprising an overpressure relief valve inparallel with said diaphragm means.
 3. A device according to claim 1,including a stop for limiting the travel of said spool and therefore themaximum opening of said first outlet port.
 4. A device according toclaim 3, wherein said stop is adjustable.
 5. A device according to claim1, wherein said intermediate chamber has a second outlet port having anopening which is varied by the piston as said spool moves and which istotally closed when the opening of the first outlet port is reduced bysaid piston.
 6. In a fuel supply circuit of an aircraft engine includingfuel injectors and a plurality of servovalves, a low pressure pump forraising the pressure of fuel from a tank from a pressure Pca to apressure Pb, and a high pressure pump and filter for further raising thepressure of the fuel to a pressure Psf at the outlet of said filter, aprocess for controlling the pressure and delivery of fuel to saidplurality of servovalves comprising the steps of: controlling the highpressure pump delivery so that, whatever the engine speed, the deliveryfrom said high pressure pump corresponds to the sum of the deliveryrequired by said fuel injectors at said engine speed and the prevailingflight conditions and the delivery which would be required by saidplurality of servovalves if all of said servovalves operatesimultaneously using the maximum delivery usable at said engine speed;placing a servovalve feed unit downstream of a fuel injector feed unitand in communication with a compartment having an opening which isvariable in dependence upon the position of a movable closure member;and, causing said closure member to move to vary said opening inresponse to opposing first and second groups of forces, said first groupof forces acting in the closing direction and comprising a resilientforce, a force proportional to the pressure Pca, and a forceproportional to a first pressure, and said second group of forces actingin the opening direction and comprising a force proportional to thepressure Pb, and a force proportional to a second pressure less than thefirst pressure and at a substantially constant difference therefrom.